The NSA convinced IBM that 56 bits was "enough".

But whereas Lucifer had a key that was 112 bits long, the DES key was shortened to 56 bits at the request of the National Security Agency.

Practical UNIX & Internet Security

In the development of DES, NSA convinced IBM that a reduced key size was sufficient

Data Encryption Standard - Wikipedia

The NSA made two changes to DES: It tweaked the algorithm, and it cut the key size by more than half.

The Legacy of DES - Bruce Schneier

The NSA convinced IBM that 56 bits was "enough":

But whereas Lucifer had a key that was 112 bits long, the DES key was shortened to 56 bits at the request of the National Security Agency.

from Practical UNIX & Internet Security

In the development of DES, NSA convinced IBM that a reduced key size was sufficient

from Data Encryption Standard - Wikipedia

The NSA made two changes to DES: It tweaked the algorithm, and it cut the key size by more than half.

from The Legacy of DES - Bruce Schneier